Multiple load control for variable frequency drive harmonic mitigation

ABSTRACT

Apparatuses, methods, and systems of multiple load control for variable frequency drive harmonic mitigation are disclosed. An exemplary embodiment includes a system including a plurality of loads including at least one compressor configured to compress refrigerant. The plurality of loads are driven by a plurality of electric motors. The plurality of electric motors are driven by a plurality of variable frequency drives. The plurality of variable frequency drives are electrically coupled to an AC source. In operation a desired cooling capacity of the system is determined, a harmonic mitigation control parameter is determined, and output of one or more of the variable frequency drives is activated, deactivated, adjusted or modulated to reduce the input harmonics and provide the desired cooling capacity.

BACKGROUND

A number of efforts have been made to mitigate power supply line input harmonics in variable frequency drive systems, including link chokes, line reactors, multiphase techniques, harmonic filters, active harmonic mitigation techniques, and combinations of these and other efforts. While providing some benefits, these approaches suffer from a number of shortcomings including cost, complexity, efficiency, efficacy, and adaptability. These issues may be of particular interest in heating, ventilation, air conditioning, or refrigeration (HVACR) applications which may include multiple loads driven by electric motors which are in turn driven by variable frequency drives. Harmonic mitigation is needed in such systems to improve losses and avoid system damage, yet the controls for such systems typically require one or more load drives to be turned on or off, or output controlled as a function of system capacity demand, such as a commanded system cooling capacity. Heretofore these systems have faced undesirable trade-offs with respect to capacity control and harmonic mitigation. There is a significant need for the unique and inventive apparatuses, methods and systems of multiple load control for variable frequency drive harmonic mitigation disclosed herein.

DISCLOSURE

For the purposes clearly, concisely and exactly describing exemplary embodiments of the invention, the manner and process of making and using the same, and to enable the practice, making and use of the same, reference will now be made to certain exemplary embodiments, including those illustrated in the figures, and specific language will be used to describe the same. It shall be understood that no limitation of the scope of the invention is thereby created, and that the invention includes and protects such alterations, modifications, and further applications of the exemplary embodiments as would occur to one skilled in the art to which the invention relates.

SUMMARY

Unique apparatuses, methods, and systems of multiple load control for variable frequency drive power line input harmonic mitigation are disclosed. An exemplary system includes a plurality of loads including at least one compressor configured to compress refrigerant. The plurality of loads are driven by a plurality of electric motors. The plurality of electric motors are driven by a plurality of variable frequency drives. The plurality of variable frequency drives are electrically coupled to an AC source. In operation a desired cooling capacity of the system is determined, a harmonic mitigation control parameter is determined, and output of one or more of the variable frequency drives is activated, deactivated, adjusted or modulated to reduce the input harmonics and provide the desired cooling capacity. Further embodiments, forms, objects, features, advantages, aspects, and benefits shall become apparent from the following description and drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic illustration of an exemplary HVACR system.

FIG. 2 is a schematic illustration of an exemplary variable frequency drive and permanent magnet motor.

FIG. 3 is a schematic illustration of an exemplary HVACR system.

FIG. 4 is a schematic illustration of an exemplary HVACR system.

FIG. 5 is a schematic illustration of an exemplary HVACR system.

FIG. 6 is a schematic illustration of several exemplary HVACR system configurations.

FIG. 7 is a flow diagram illustrating an exemplary control process.

DETAILED DESCRIPTION

With reference to FIG. 1 there is illustrated an exemplary HVACR system 100 which includes a refrigerant loop comprising compressors 110 and 111, a condenser 120, and an evaporator 130. Refrigerant flows through system 100 in a closed loop from compressors 110 and 111 to condenser 120 to evaporator 130 and back to compressors 110 and 111. Various embodiments may also include additional refrigerant loop elements including, for example, valves for controlling refrigerant flow, refrigerant filters, economizers, oil separators and/or cooling components and flow paths for various system components.

Compressor 110 is driven by a drive unit 150 including a permanent magnet electric motor 170 which is driven by a variable frequency drive 155. In the illustrated embodiment, variable frequency drive 155 is configured to output a three-phase PWM drive signal, and motor 170 is a surface magnet permanent magnet motor. Use of other types and configurations of variable frequency drives and electric motors such as interior magnet permanent magnet motors, reluctance motors, or inductance motors are also contemplated. Compressor 111 is driven by a drive unit 151 including a permanent magnet electric motor 171 which is driven by a variable frequency drive 156. In the illustrated embodiment, variable frequency drive 156 is configured to output a three-phase PWM drive signal, and motor 171 is a surface magnet permanent magnet motor. Use of other types and configurations of variable frequency drives and electric motors such as interior magnet permanent magnet motors, reluctance motors, or inductance motors are also contemplated. It shall be appreciated that the principles and techniques disclosed herein may be applied to a broad variety of drive and motor configurations, systems and subsystems including those further described herein below. It shall be further appreciated that the same applies to a number of additional or alternate controllers, control modules or control units, including but not limited to those described elsewhere herein.

Condenser 120 is configured to transfer heat from compressed refrigerant received from compressor 110. In the illustrated embodiment condenser 120 is a water cooled condenser which receives cooling water at an inlet 121, transfers heat from the refrigerant to the cooling water, and outputs cooling water at an output 122. It is also contemplated that other types of condensers may be utilized, for example, air cooled condensers or evaporative condensers. It shall further be appreciated that references herein to water include water solutions comprising additional constituents unless otherwise limited.

Evaporator 130 is configured to receive refrigerant from condenser 120, expand the received refrigerant to decrease its temperature and transfer heat from a cooled medium to the refrigerant. In the illustrated embodiment evaporator 130 is configured as a water chiller which receives water provided to an inlet 131, transfers heat from the water to the refrigerant, and outputs chilled water at an outlet 132. It is contemplated that a number of particular types of evaporators may be utilized, including dry expansion evaporators, flooded type evaporators, bare tube evaporators, plate surface evaporators, and finned evaporators among others.

HVACR system 100 further includes a controller 160 which outputs control signals to variable frequency drives 155 and 156 to control operation of motors 170 and 171 and compressors 110 and 111. Controller 160 also receives information about the operation of drive units 150 and 151. In exemplary embodiments controller 160 receives information relating to motor current, motor terminal voltage, and/or other operational characteristics of the motor. It shall be appreciated that the controls, control routines, and control modules described herein may be implemented using hardware, software, firmware and various combinations thereof and may utilize executable instructions stored in a non-transitory computer readable medium or multiple non-transitory computer readable media. It shall further be understood that controller 160 may be provided in various forms and may include a number of hardware and software modules and components such as those disclosed herein below.

With reference to FIG. 2 there is illustrated an exemplary circuit diagram for a variable frequency motor drive 200. It shall be appreciated that drive 200 provides one example of the details of a motor drive which may be utilized in connection with the systems disclosed herein as well as other systems utilizing the disclosed techniques. Drive 200 is connected to a power source 210, for example, a 400/480 VAC utility power supply which provides three-phase AC power to line filter module 220. Line filter module 220 is configured to provide harmonic damping to mitigate losses which can arise from harmonic feedback from drive components to power source 210. Line filter module 220 outputs three-phase AC power to a rectifier 290 which converts the AC power to DC power and provides the DC power to a DC bus 291. DC bus 291 is preferably a film capacitor-cased bus which includes one or more film capacitors electrically coupled between positive and negative bus rails. DC bus 291 is connected to inverter 280. For clarity of illustration and description, rectifier 290, DC bus 291, and inverter 280 are shown as discrete elements. It shall be appreciated, however, that two or more of these components may be provided in a common module, board or board assembly which may also include a variety of additional circuitry and components. It shall be further understood that, in addition to the illustrated 6-pulse rectifier, other multiple pulse rectifiers such as 12-pulse, 18-pulse, 24-pulse or 30-pulse rectifiers may be utilized along with phase shifting transformers providing appropriate phase inputs for 6-pulse 12-pulse, 18-pulse, 24-pulse, or 30-pulse operation. High pulse orders may also be utilized.

Inverter module 280 includes switches 285, 286 and 287 which are connected to the positive and negative rails of DC bus 291. Switches 285, 286 and 287 are preferably configured as IGBT and diode based switches, but may also utilize other types of power electronics switching components such as power MOSFETs or other electrical switching devices. Switches 285, 286 and 287 provide output to motor terminals 275, 276 and 277. Current sensors 281, 282 and 283 are configured to detect current flowing from inverter module 280 to motor 270 and send current information to ID module 293. Voltage sensors are also operatively coupled with motor terminals 275, 276 and 277 and configured to provide voltage information from the motor terminals to ID module 293.

ID module 293 includes burden resistors used in connection with current sensing to set the scaling on current signals ultimately provided to analog to digital converters for further processing. ID module 293 tells the VFD what size it is (i.e. what type of scaling to use on current post ADC) using identification bits which are set in hardware on the ID module 293. ID module 293 also outputs current and voltage information to gate drive module 250 and also provides identification information to gate drive module 250 which identifies the type and size of the load to which gate drive module 250 is connected. ID module 293 may also provide current sensing power supply status information to gate drive module 250. ID module 293 may also provide scaling functionality for other parameters such as voltage or flux signals in other embodiments.

Gate drive module 250 provides sensed current and voltage information to analog to digital converter inputs of DSP module 260. DSP module 260 processes the sensed current and voltage information and also provides control signals to gate drive module 250 which control signals gate drive module 250 to output voltages to boost modules 251, 252 and 253, which in turn output boosted voltages to switches 285, 286 and 287. The signals provided to switches 285, 286 and 287 in turn control the output provided to terminals 275, 276 and 277 of motor 270.

Motor 270 includes a stator 271, a rotor 273, and an air gap 272 between the rotor and the stator. Motor terminals 275, 276 and 277 are connected to windings provided in stator 271. Rotor 273 includes a plurality of permanent magnets 274. In the illustrated embodiment magnets 274 are configured as surface permanent magnets positioned about the circumference of rotor 273. The rotor is typically constructed using the permanent magnets in such a way as essentially a constant magnetic flux is present at the surface of the rotor. In operation with rotation of the rotor, the electrical conductors forming the windings in the stator are disposed to produce a sinusoidal flux linkage. Other embodiments also contemplate the use of other magnet configurations such as interior magnet configurations as well as inductance motor configurations, reluctance motor configurations and other non-permanent magnet configurations.

With reference to FIG. 3 there is illustrated an exemplary HVACR system 300. System 300 includes a variable frequency drive 360 including an AC/DC converter or rectifier 361, a DC link or bus 362, and a DC/AC converter or inverter 363. Variable frequency drive 360 is configured to drive load 364 which may be one of several different types of loads of an HVACR system including, for example, an electric motor driving a refrigerant compressor, an electric motor driving a condenser load such as a pump or one or more fans, an electric motor driving a fan deck, or an electric motor driving a variety of other mechanical loads. The output of variable frequency drive 360 is controlled by control signals provided from control unit 320.

Control unit 320 also provides control signals to variable frequency drive 350 which includes an AC/DC converter or rectifier 351, DC link or bus 352, and DC/AC converter or inverter 353. Variable frequency drive 350 provides output to load 354 which may be one of a number of different types of loads of an HVACR system including, for example, those described above in connection with load 364. While system 300 is illustrated as including two loads, it shall be appreciated that a greater number of loads and corresponding drives may be present in various embodiments.

Variable frequency drive 350 receives input from transformer 330 which may be a phase shifting transformer such as an autotransformer or an isolation transformer. Variable frequency drive 360 receives input from transformer 340 which may be a phase shifting transformer such as an autotransformer or an isolation transformer. Transformers 330 and 340 receive input from line source 310. In the illustrated embodiment line source 310 is a three-phase, 400/480 VAC power source, though it shall be appreciated that other power sources may be used in various embodiments. Transformers 330 and 340 may be configured as 6-pulse, 9-pulse, 12-pulse or other multi-pulse transformers. The rectifiers or AC/DC converters 351 and 352 may be correspondingly configured as 6-pulse, 9-pulse, 12-pulse, or other multi-pulse configurations. These configurations provide a quasi-12, -18, -24 or other quasi-multi-pulse front ends. These front ends are preferably configured to provide harmonic cancellation when the respective drives and loads are operating according to the controls described herein, for example, by phase shifting or staggering the harmonic content between two or more operating loads.

In certain exemplary embodiments, a monitoring device 311 is configured to sense input information (such as current information, voltage information, power information or harmonic information) at a point common to all loads of system 300 and provide this information to control unit 320. It is contemplated that monitoring device 311 may take a number of forms. A current sensor may be utilized when input current harmonic cancellation is desired. A current sensor and voltage sensor may both be utilized when limits of load cancellation are to be understood with regard to sources of harmonic distortion other than load imbalance, sometimes referred to as external sources or external influences. It should be appreciated that when load harmonic currents are due to input voltage distortion, load imbalance distortion is preferably distinguished from external sources or the system will attempt to compensate for the condition of the external source or sources. The sensed harmonic information is preferably used to determine input total demand distortion currents (TDD) which is a function of the rated load amps (RLA) of the overall system 300 including the individual contributions of each load subsystem. In other embodiments, the sensed harmonic current information may be utilized to estimate terminal voltage total harmonic distortion (THD) which may be compared to actual measured voltage THD. The harmonic distortion determined from the measured harmonic information may be utilized by control logic to provide load balancing effective to mitigate input harmonics. It shall be appreciated that the control logic implementing these functionalities may be provided in control unit 320 and/or in additional or alternate control units and may be implemented in accordance with the further examples described herein.

In additional exemplary embodiments preconfigured open loop controls may be utilized in which operates balancing logic without the need to measure input information. The control logic in such embodiments may be preconfigured according to empirically or theoretically derived parameters which are based upon observations or predictions about the relative load balances which produce desired harmonic mitigation.

With reference to FIG. 4 there is illustrated an exemplary HVACR system 400. System 400 includes a refrigerant circuit 402 which includes a refrigerant compressor 410, a condenser 420, and an evaporator 430. Compressor 410 provides compressed refrigerant to condenser 420 which operates to transfer heat from the compressed refrigerant to a cooling medium. In the illustrated embodiment the cooing medium is ambient air which is circulated across condenser 420 by one or more fans of fan deck 423. Cooled compressed refrigerant is provided from condenser 420 to evaporator 430 which expands the refrigerant to transfer heat from a cooled medium provided at input 432 and output at output 433. From evaporator 430 refrigerant returns to compressor 410.

System 400 includes a refrigerant circuit 404 which includes a compressor 411, a condenser 421, and an evaporator 431. Compressor 411 provides compressed refrigerant to condenser 421 which operates to transfer heat from the compressed refrigerant to a cooling medium. In the illustrated embodiment the cooing medium is ambient air which is circulated across condenser 421 by one or more fans of fan deck 423. Cooled compressed refrigerant is provided from condenser 421 to evaporator 431 which expands the refrigerant to transfer heat from a cooled medium provided from output 433 of condenser 430 and output at output 434 of evaporator 431. From evaporator 431 refrigerant returns to compressor 411. In the illustrated embodiment, evaporators 430 and 431 are configured to transfer heat from a common circuit of cooled medium. The cooled medium input 432 to condenser 430 is provided as output 433 to condenser 431 which provides further output 434. It shall be appreciated that other configurations including separate cooled media circuits may be utilized in various embodiments. Fan deck 423 is one example of a load configured to circulate a cooling medium across condensers 420 and 421. Fan deck 423 may include one or more fans configured to circulate ambient air across one or both of condensers 420 and 421. It shall be appreciated that other configurations and other cooling media may also be utilized in various embodiments.

System 400 also includes variable frequency drives 450, 451 and 452 which are configured to drive electric motors 470, 471 and 472, respectively. Electric motors 470, 471 and 472 are configured to drive compressor 410, compressor 411 and one or more fans of fan deck 423, respectively. The output of variable frequency drives 450, 451 and 452 are controlled by control signals provided from control unit 460. Variable frequency drives 450, 451 and 452 receive input from transformers 449, 447 and 445, respectively, which may be phase shifting transformers such as autotransformers or isolation transformers. Transformers 449, 447 and 445 receive input from line source 410. In the illustrated embodiment line source 410 is a three-phase, 400/480 VAC power source, though it shall be appreciated that other power sources may be used in various embodiments. Transformers 449, 447 and 445 may be configured as 6-pulse, 9-pulse, 12-pulse or other multi-pulse transformers. The rectifiers or AC/DC converters of variable frequency drives 450, 451 and 452 may be correspondingly configured as 6-pulse, 9-pulse, 12-pulse, or other multi-pulse configurations. These configurations provide a quasi-12, -18, -24 or other quasi-multi-pulse front ends. These front ends are preferably configured to provide harmonic cancellation when the respective drives and loads are operating according to the controls described herein, for example, by phase shifting or staggering the harmonic content between two or more operating loads.

HVACR system 400 further includes a control unit 460 which outputs control signals to variable frequency drives 450, 451 and 452 to control operation of motors 470, 471 and 472. In certain exemplary embodiments, a monitoring device 499 is configured to sense input information (such as current information, voltage information, power information or harmonic information) and provide the same to control unit 460 and may be provided in any of a variety of forms including those described above in connection with monitoring device 311. The sensed harmonic information is preferably used to determine input current total demand distortion (TDD) which is a function of the rated load amps (RLA) of the overall system 400. In other embodiments, the sensed harmonic current information may be utilized to estimate terminal voltage total harmonic distortion (THD) which may be compared to actual measured voltage THD. Control unit 460 also receives information about the operation of drive units 470, 471 and 472. In exemplary embodiments control unit 460 receives information relating to motor current, motor terminal voltage, and/or other operational characteristics of the motors. It shall be appreciated that the controls described herein may be implemented using hardware, software, firmware and various combinations thereof and may utilize executable instructions stored in a non-transitory computer readable medium or multiple non-transitory computer readable media. It shall further be understood that controller 460 may be provided in various forms and may include a number of hardware and software modules and components such as those disclosed herein.

In additional exemplary embodiments preconfigured open loop controls may be utilized in which operates balancing logic without the need to measure input information. The control logic in such embodiments may be preconfigured according to empirically or theoretically derived parameters which are based upon observations or predictions about the relative load balances which produce desired harmonic mitigation. With reference to FIG. 5 there is illustrated an exemplary HVACR system 500. System 500 includes a plurality of variable frequency drives 520, 530, 540 and 550 which are configured to drive loads 521, 531, 541 and 551, respectively, each of which may be any of several different types of loads of an HVACR system including, for example, an electric motor driving a refrigerant compressor, an electric motor driving a condenser load such as a pump or one or more fans, an electric motor driving a fan deck, or an electric motor driving a variety of other mechanical loads. The output of variable frequency drives 520, 530, 540 and 550 are controlled by control signals provided from control unit 560. While system 500 is illustrated as including four loads, it shall be appreciated that greater or lesser numbers of loads and corresponding drives may be present in various embodiments as indicated by ellipsis 590. A monitoring device 511 is configured to sense input information and provide the same to control unit 320 and may be provided in a number of forms including those described above in connection with monitoring device 311.

Variable frequency drives 520, 530, 540 and 550 receive input from transformers 519, 529, 539, and 549, respectively, which may be phase shifting transformers such as autotransformers or isolation transformers. Transformers 519, 529, 539, and 549 may be configured as 6-pulse, 9-pulse, 12-pulse or other multi-pulse transformers. The rectifiers or AC/DC converters 351 and 352 may be correspondingly configured as 6-pulse, 9-pulse, 12-pulse, or other multi-pulse configurations. These configurations provide quasi-multi-pulse front ends. These to front ends are preferably configured to provide harmonic cancellation when the respective drives and loads are operating according to the controls described herein, for example, by phase shifting or staggering the harmonic content between two or more operating loads.

Control unit 560 outputs control signals to variable frequency drives 520, 530, 540 and 550 to control the output they provide to their respective loads. In certain exemplary embodiments monitoring device 511 is configured to sense input information (such as current information, voltage information, power information or harmonic information) and provide the same to control unit 560. The sensed harmonic information may be any of a number of types including those described above in connection with FIGS. 3 and 4. Control unit 560 also receives information about the operation of drive units 520, 530, 540 and 550. In exemplary embodiments control unit 560 receives information relating to motor current, motor terminal voltage, and/or other operational characteristics of electric motors of the respective loads. It shall be appreciated that the controls described herein may be implemented using hardware, software, firmware and various combinations thereof and may utilize executable instructions stored in a non-transitory computer readable medium or multiple non-transitory computer readable media. It shall further be understood that controller 560 may be provided in various forms and may include a number of hardware and software modules and components such as those disclosed herein.

In additional exemplary embodiments preconfigured open loop controls may be utilized in which operates balancing logic without the need to measure input information. The control logic in such embodiments may be preconfigured according to empirically or theoretically derived parameters which are based upon observations or predictions about the relative load balances which produce desired harmonic mitigation. With reference to FIG. 6 there are illustrated several exemplary HVACR systems including two or more subsystems which are coupled such that the individual cooling capacities contribute to providing the overall system cooling capacity. In these and other such systems, load requirements can be distributed amongst the subsystems according to multiple performance criteria, such as efficiency, distribution of temperature stress or differentials, and subsystem input harmonic currents. As indicated by the exemplary embodiments illustrated, the multiple loads may be coupled in parallel, in series, or any combination thereof as required to satisfy overall system output conditions.

System 610 illustrates one example of a parallel system configuration. System 610 includes a system input 601 from which a cooled medium is provided to sub-systems 602, 604, and 606 in parallel. Sub-systems 602, 604, and 606 may be selectively operated and controlled to provide various degrees of cooling to the cooled medium and collectively output to system output 609. As indicated by ellipsis A, it is contemplated any number of two or more subsystems may be provided in a parallel configuration.

System 620 illustrates one example of a series system configuration. System 620 includes a system input 611 from which a cooled medium is provided to subsystems 612, 614, and 616 in series. Sub-systems 612, 614, and 616 may be selectively operated and controlled to provide various degrees of cooling to the cooled medium and collectively output to system output 619. System 620 may be utilized in a variety of applications including, for example, those needing a greater degree of cooling than can be provided by a single subsystem due to practical capacity limitations. As indicated by ellipsis B, it is contemplated any number of two or more subsystems may be provided in a series configuration.

System 630 illustrates one example of a combined parallel/series system configuration. System 630 includes a system input 621 from which a cooled medium is provided to sub-systems 622, 626, and 628 in parallel and from subsystem 622 to subsystem 624 in series. Sub-systems 622, 624, 626 and 628 may be selectively operated and controlled to provide various degrees of cooling to the cooled medium and collectively output to system output 629. As indicated by ellipses C, D and E, it is contemplated various numbers of multiple subsystems may be provided in parallel/series configurations ranging upward from two units in series in parallel with a third.

In systems 610, 620 and 630, the subsystems are controlled by one or more controllers such as those disclosed herein, and the input lines to each of the subsystems are connected to multiphase transformers for multi-pulse rectifier service. The phase shifts of the multi-phase transformers are preferably selected to provide a symmetrical, balanced load to the total electrical supply and equal amounts of load on each phase at full load of the system. Where loads cannot be perfectly balanced, the loads may be selected to provide phase symmetry and load balance as much as is practical.

With reference to FIG. 7 there is illustrated a flow diagram of an exemplary control process 700. Process 700 is an illustrative example of a control process configured to provide optimized load operation which results in phase symmetry and electrical load balance, and rectifier system harmonics cancellation. This type of control process is useful in compensating for inherent differences in equipment load to characteristics and variation in phase impedances, which result in degradation of harmonics cancellation from ideal operation. It shall be understood that the term optimized and grammatical variants thereof is not limited to theoretical maximum or perfect optimization and includes implementations that achieve an acceptable or desired degree of optimization as dictated by the needs and design goals of particular embodiments. It shall be appreciated that the control logic, processes and techniques described herein may be implemented in a variety of manners including different combinations of software, hardware, and/or firmware which may be provided in the devices, controllers and control units disclosed herein above, for example, or distributed across multiple controllers or control units alternate to or in addition to the same.

Process 700 is started at operation 710 and proceeds to operation 720 which initializes a primary control loop for an HVACR system including a plurality of machines. From operation 720 process 700 proceeds to operation 730 which performs sensing and control operations that provide a desired or targeted system capacity requirement. From operation 730 process 700 proceeds to conditional 740 which determines whether an increase or decrease in the number of machines that are operating is desired or required. It shall be appreciated that the machines may be provided in a variety of configurations including, for example, the configurations illustrated and described herein.

If conditional 740 returns true, process 700 proceeds to operation 750 which determines a particular machine or machines to turn on or off and the initial capacity for any machine or machines which are turned on. From operation 750 process 700 proceeds to conditional 760. Likewise, if conditional 740 returns negative, process 700 proceeds from conditional 740 to conditional 760. Conditional 760 determines whether an increase or decrease in capacity of the machine or machines that are operating is required or desired. If conditional 760 returns affirmative, operation 700 proceeds to operation 770 which determines a particular machine or machines to increase or decrease in capacity and the magnitude of the increase or decrease in capacity for the machine or machines. In certain embodiments operation 770 may receive input from operation 765 which provides information relating to system input conditions, such as the input parameters and characteristic described herein. Such embodiments may utilize various closed loop controls, including those described herein, though it shall be appreciated that open loop controls are equally applicable and utilizable in other embodiments which would not receive or include input such as that of operation 765. From operation 770 process 700 returns to operation 730. Likewise, if conditional 770 returns negative, process 700 returns to operation 730.

Process 700 is an illustrative example of a control technique in which primary capacity control logic and load determination and balancing logic operate in concert to satisfy the overall commanded, desired or targeted capacity. In process 700 the primary capacity control logic is responsible for determining whether changes in machine staging or capacity are warranted to satisfy overall system performance requirements, and the load determination and balancing logic is responsible for identifying the particular machine or machines that should be turned on or off, or whose capacity should be adjusted as well as the initial capacity of a machine or machines that are turned on and an adjusted capacity of a machine or machines that are adjusted.

Operations 740 and 760 may be provided using a number of methods, forms and implementations. It shall be appreciated that the details of implementation in a given embodiments may be determined by the typical control methods for individual machines as practiced using methods established for a particular application. Adjustment for load biasing factors, such as a relative efficiency factor, may be included. Furthermore, control of balance for harmonic cancellation can be done in either open loop or closed loop forms.

Operation 750 may be provided using a number of methods, forms and implementations. An exemplary method of computing a designated incremental machine or machines is to implement a search algorithm, which computes the predicted capacity and balance results of all viable machine combinations using load staging logic. This can be done by totaling the incremental available capacity for each phase balance portion, and storing the calculated total predicted balance for each configuration. For systems with large variation beyond the expected operating tolerance, an adaptive memory matrix can be used which stores the proportional load balance experienced under running conditions, experienced under the active balancing control algorithm. The optimal machine or other load to increment is then selected from the search results.

A number of forms of load staging logic are contemplated. In some exemplary forms, the load staging logic implements a load designation algorithm which selects specific sequenced loads to add or subtract according to the primary capacity control requirement or target based upon a determination or estimation of which load addition(s) or subtraction(s) will result in the greatest capability to accomplish electrical system balance and cancellation of line input harmonics. Some exemplary forms include a load balancing algorithm to distribute to load between all coupled operating units, to optimize the cancellation of line input harmonics. Some exemplary forms utilize an algorithm executable to determine a combination of operating machine subsystems which will result in a minimized (or acceptably minimized) increment of capacity which will satisfy the expected demand of a primary cooling loop, while meeting the constraint of providing the closest capacity balancing capability. Some exemplary forms utilize a computation method which calculates balance results based on a stored matrix of machine configuration for the system design, designating phase balance alignment and capacity influence on balancing (% system capacity produced for the primary loop). For staged systems, a matrix is preferably developed or computed for each stage configuration. Some exemplary forms fixed sequencing matrix, pre-calculated using the balance algorithm mentioned above. The fixed matrix method may be preferred for simple systems, where the number of matrix combinations of machines is limited.

Table 1 below illustrates an example of initial capacity allocations for a new configuration of operating machines which is initiated by supervisory capacity staging logic as well as application of an efficiency correction based on machine efficiency information.

TABLE 1 Machine and Initial Initial Maximum Capacity Capacity Capacity as Without With Percent of Total Staging Next State Efficiency Efficiency System Capacity Initialization Desired Corrections Corrections Machine A = 50% Operating at Operating 25% 30% 100% machine capacity, 50% system capacity Machine B = 25% Not operating Operating 25% 20% Machine C = 25% Not operating Not  0%  0% operating

Table 1 illustrates the case of stage increase from one machine to two machines operating. Initially, only Machine A is operating at 100% capacity to provide 50% of system capacity. To increase capacity, another machine must be brought into operation. The next stage desired, as indicated by a the system staging logic, is a machine capable of an additional 25% of system load. To create staging with minimal capacity disturbance, the additional machine must be started and loaded to approximately equal the prior load state. The initial staging capacity is calculated to provide a target setting for the new combination of running machines. To optimize system operation, if desired, an efficiency correction can be applied to the initial settings, and used in subsequent capacity modifications as an estimate of optimal capacity ratio between running machines.

Operation 770 may be provided using a number of methods, forms and implementations. In general, the controls and logic for allocation of capacity among two or more drives, machines or subsystems are preferably configured to allocate capacity to provide an optimized balance of load, and minimize line input harmonic currents. Adjustment for load biasing factors, such as a relative efficiency factor, may also be provided. Controls of balance for harmonic cancellation can be done in either open loop or closed loop forms. It shall be appreciated that specific implementations for a given system may be influenced by and configured in accordance with the control methods for which individual machines are configured.

In some exemplary methods, a primary control logic determines whether there is a need for increase or decrease in the number of machines operating based upon staging logic and efficiency optimization considerations. The load designation logic then determines which subsystem(s) should be targeted for load capacity increase(s) or decrease(s). The capacity regulation is determined by individual machine control techniques, which may be either an operating control setpoint (such as temperature), or a capacity control setpoint. The balance control algorithm directs capacity control adjustments to either individual machines, or to the machines as a collective whole, if the configuration obtains balance with enough precision to accept incremental changes in all machines simultaneously.

In some exemplary embodiments for systems configured to accept incremental commands to all operating machines simultaneously, as machines are staged initial capacity is distributed to accomplish smooth capacity changes with minimal disturbance. This may be accomplished by calculating proportional load balance, according to the optimal machine operation found by using the search algorithm mentioned previously. After initialization, adjustment of capacity is done by incremental adjustment of control to all operating machines, until capacity saturation is reached, and staging adjustment is needed.

Some exemplary embodiments are configured to accept incremental commands to individual machines. In order to create a desired control resolution, control capacity incremental commands preferably done to maintain capacity balance within tolerance of the system and with tolerance of the capacity balance result to line input harmonic currents. This may be done by directing capacity adjustment commands created by the primary control to designated machines identified by a search routine such as the routines described herein. Initialization may be performed using the same or similar technique as above, to minimize staging disturbances. After staging decisions are made, initial capacity is re-distributed amongst the operating machines to accomplish smooth capacity changes with minimal disturbance. This is done by calculating proportional load balance, according to the optimal machine operation found by using the search algorithm mentioned previously. The requirement for staging is determined by incremental loading capacity saturation, invoking the primary control staging logic and balancing algorithm directive.

Table 2 below illustrates an example of capacity change logic for a system including three machines with an initial staging with efficiency correction. For modification of machine capacities, the configuration initial capacity is used. The configuration may account for efficiency corrections and/or harmonic mitigation corrections. For example, in closed loop controls the measured common point information may be used to affect the balance choice for capacity modification to optimally reduce harmonic content. This may be accomplished in a similar manner to modification of the balance target for efficiency. In some forms, the measurement information may be compared to reference load balance information to direct which load to adjust for balance by adjusting a target ratio. Depending upon the various system performance goals and harmonic mitigation goals that may coexist in various operative states, the balance between these optimization goals may be adjusted relative to one another to meet the overall performance goals of the system. This technique may be implemented in a dynamic supervisory control implemented in any of the hardware, software, and/or firmware implementations disclosed herein as well as alternative systems. This provides a reference ratio which is used to designate which machine capacity is adjusted. The selection is done such that the new value provides the closest possible ratio to the reference initial capacity.

TABLE 2 Machine and Initial Designated Designated Maximum Capacity With Current Machine for Machine for Capacity as Efficiency Capacity 1% Capacity 1% Capacity Percent of Corrections After Increase to Decrease to Total System from Staging Capacity Achieve Target Achieve Target Capacity Logic Change Capacity Ratio Capacity Ratio 50% 30% 32% X 25% 20% 23% X 25% 0 0

Control processes such as process 700 may be utilized in connection with a number of applications. Some applications include measuring input harmonics and calibration of balancing adjustments. These techniques may utilize harmonic measurements at common point of power line input to the system, for example, as described above. The input current harmonics may be measured for monitoring system operation, and for calibration of the balance adjustment algorithms. Balance may be obtained by design of the system within manufacturing tolerances, typically +/−10%. To allow adjustment for these variations, and also to provide some degree of closed loop control, the degree of harmonic cancellation can be monitored for each system configuration, and where the harmonics are minimized, the capacity ratios may be memorized and applied as the corrected target ratio for that operating configuration. Some applications include discriminating or identifying external harmonic influences. A current sensor and a voltage sensor may be utilized together to determine the limits of harmonic cancellation through load adjustment with regard to other sources of harmonic distortion. Input voltage TDD may be determined and compared to a predicted voltage TDD. Contributions of external sources of harmonic distortion can be identified based upon differences between the determined and predicted voltage TDD.

It shall be understood that the exemplary embodiments summarized and described in detail above and illustrated in the figures are illustrative and not limiting or restrictive. Only the presently preferred embodiments have been shown and described and all changes and modifications that come within the scope of the invention are to be protected. It shall be appreciated that the embodiments and forms described below may be combined in certain instances and may be exclusive of one another in other instances. Likewise, it shall be appreciated that the embodiments and forms described below may or may not be combined with other aspects and features disclosed elsewhere herein. It should be understood that various features and aspects of the embodiments described above may not be necessary and embodiments lacking the same are also protected. In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary. 

1. A method comprising: operating an HVACR system including a plurality of loads including at least one compressor configured to compress refrigerant, the plurality of loads being driven by a plurality of electric motors, the plurality of electric motors being driven by a plurality of variable frequency drives, the plurality of variable frequency drives being electrically coupled to an AC source; determining a desired cooling capacity of the system; measuring input information associated with the variable frequency drives; and adjusting output of one or more of the variable frequency drives to reduce input harmonics to the variable frequency drives and provide the desired system cooling capacity.
 2. A method according to claim 1 wherein the plurality of loads include a plurality of compressors configured to compress refrigerant. 3.-7. (canceled)
 8. A method according to claim 1 wherein the plurality of loads include the compressor configured to compress refrigerant and a condenser fan drive.
 9. A method according to claim 1 further comprising determining input current TDD from the measured input information.
 10. A method according to claim 9 further comprising determining a terminal voltage THD and comparing the determined terminal voltage THD to a measured terminal voltage THD to evaluate efficacy of harmonic mitigation. 12-15. (canceled)
 16. A system comprising: a plurality of electric motors configured to drive a respective plurality of loads including at least one refrigerant compressor; a plurality of variable frequency drives configured to drive the plurality of electric motors; and a controller configured to control output of the variable frequency drives; wherein the controller is configured to: determine a target cooling capacity of the system, measure input current harmonics associated with the variable frequency drives, and adjust output of one or more of the variable frequency drives to mitigate the input current harmonics while providing the target cooling capacity.
 17. A system according to claim 16 wherein the variable frequency drives each include a rectifier electrically coupled to an AC source, a DC link electrically coupled to the rectifier and an inverter electrically coupled to the DC link. 18.-24. (canceled)
 25. A system according to claim 16 wherein the controller is configured to determine input current TDD and input voltage THD.
 26. A system according to claim 25 wherein the controller is configured to determine contributions of load imbalance and other sources of distortion to net harmonic distortion utilizing the input current TDD and the input voltage THD.
 27. A system according to claim 16 wherein the controller is configured to adjust the variable frequency drives collectively in response to an increase or decrease in the target cooling capacity.
 28. A system according to claim 16 wherein the controller is configured to adjust a particular one of the variable frequency drives selected to optimize harmonic mitigation in response to an increase or decrease in the target cooling capacity.
 29. A system according to claim 16 wherein the controller is configured to operate capacity control logic to determine a capacity increase or decrease and operate load balancing logic to determine one or more loads to adjust to provide the increase or decrease while mitigating system input harmonics.
 30. A system according to claim 29 wherein the balancing logic is configured to calculate balance results based on a stored matrix of machine configuration for the system design designating phase balance alignment and capacity influence on balancing.
 31. A system according to claim 29 wherein the balancing logic is configured to utilize a fixed sequencing matrix, pre-calculated using a balancing algorithm.
 32. A system according to claim 16 wherein the controller is configured to determine whether to turn on or off one or more of the electric motors to meet the target cooling capacity and to select at least one specific machine and determine its initial capacity and optimize the harmonic mitigation.
 33. A system according to claim 16 wherein the controller is configured to determine whether to change capacity of one or more of the electric motors which are operating and to select at least one specific electric motor and at least one capacity magnitude change to meet the target cooling capacity and optimize the harmonic mitigation. 34.-43. (canceled)
 44. A method comprising: operating an HVACR system including a plurality of loads including at least one compressor configured to compress refrigerant, the plurality of loads being driven by a plurality of electric motors, the plurality of electric motors being driven by a plurality of variable frequency drives, the plurality of variable frequency drives being electrically coupled to an AC source; determining a desired cooling capacity of the system; determining an operating configuration of the variable frequency drives configured to provide desired input harmonic mitigation; and controlling one or more of the variable frequency drives provide the desired input harmonic mitigation and provide the desired system cooling capacity. 45.-51. (canceled)
 52. A method according to claim 44 further comprising determining input current TDD from the measured input information.
 53. A method according to claim 52 further comprising determining a terminal voltage THD and comparing the determined terminal voltage THD to a measured terminal voltage THD to evaluate efficacy of harmonic mitigation. 54.-62. (canceled)
 63. A method according to claim 44 wherein the controlling comprises one of: adjusting output to the plurality of loads collective to increase or decrease overall capacity; adjusting output to a subset of the plurality of loads to increase or decrease overall capacity; increasing or decreasing capacity of at least one of the plurality of loads which is operating; and turning on or off at least one of the plurality of loads. 